March 17, 2008

On the night of July 7th, 1958 the world’s largest Tsunami engorged Alaska's Lituya bay, located about 250 miles west of Juneau. It was 1,700 feet or 520 meters, almost twice the height of the Eiffel Tower.

The Tsunami was triggered by a magnitude 8.3 earthquake caused an enormous landslide along the Fairweather Fault. The resulting crash of rock into water, caused the largest wall of water in human history. The deadly wave hurtled at jet speeds and wiped out everything within a four mile radius.

Fortunately Lituya Bay was virtually uninhabited, otherwise it would
have caused unprecedented destruction, far greater than the tsunami
that struck Thailand in 2004.

At the time of the colossal wave, there were only three fishing boats
anchored in the bay and amazingly only one sank, with two people losing
their lives. The other boats were able to surf the crest of the tsunami.

The Science Channel and Dr George PC quoted one of the survivors Howard
G. Ulrich in a recent article about the tsnumami. Ulrich heard the
sound of the enormous wave ripping through the land and obscuring the
sky, he reportedly said to his 8-year-old child “Son…it’s time to pray.”

Can a similar tsunami strike the westcoast of the U.S. again? Geological evidence makes it almost a certainty -the region is the heart of the world's most active sesmic zone: the Pacific Rim of Fire.

On January the 26th, 1700, sometime around 9 in the evening local time,
the Juan de Fuca segment of the planet beneath the ocean in the Pacific
Northwest moved. Suddenly. It slipped some 60 feet eastward beneath the
North American plate, and caused a monster quake of approximate
magnitude 9. It set in motion tsunamis that struck the coast of North
America and traveled to the shores of Japan.

Researchers
believe that these megaquakes occur every 400 to 500 years or so.

Kim Olsen of SDSU and his team created a supercomputer-powered
“virtual earthquake” program that allowed them to recreate such an
earthquake. This program encompassed the work of scientists from SDSU,
San Diego Supercomputer Center at UC San Diego and the U.S. Geological
Survey.

In addition, to ensure that the entire representation of what could
happen is accurate, William Stephenson of the USGS worked with Olsen
and Andreas Geisselmeyer from Ulm University in Germany to create an
accurate representation of the earth’s subsurface layering in that
area. This “velocity model” – the first of its kind – expresses how the
structure will bend, reflect, and change in size and direction.

Naturally, what they learnt didn’t necessarily send anyone home to bed
with warm fuzzy feelings of safety (although Andreas is probably
feeling pretty cozy over in Germany).

Their scenario depicted a rupture beginning in the north and
propagating toward the south along the 600-mile long Cascadia
Subduction Zone (an area where two tectonic plates move towards one
another, forcing one to slide beneath the other). In their scenario,
the ground moved about 1.5 feet per second in Seattle, nearly 6 inches
per second in Tacoma, Olympia and Vancouver, and 3 inches in Portland,
Oregon.

“We also found that these high ground velocities were accompanied by
significant low-frequency shaking, like what you feel in a roller
coaster, that lasted as long as five minutes – and that’s a long time,”
said Olsen.

“One thing these studies will hopefully do is to raise awareness of the
possibility of megathrust earthquakes happening at any given time in
the Pacific Northwest,” Olsen added. “Because these events will tend to
occur several hundred kilometers from major cities, the study also
implies that the region could benefit from an early warning system that
can allow time for protective actions before the brunt of the shaking
starts.”

Region specific, this is bad news for the North West for two reasons;
one, because the combined long-duration shaking and high ground
velocities raise the possibility that such an earthquake could inflict
major damage on downtown Seattle; and two, areas like Seattle, Tacoma
and Olympia sit on top of sediment filled geological basins, thus,
amplifying the waves generated by major earthquakes.

Reason one why scientists bother running these simulations. Reason
number two: “The information from these simulations can also play a
role in research into the hazards posed by large tsunamis, which can
originate from such megathrust earthquakes like the ones generated in
the 2004 Sumatra-Andeman earthquake in Indonesia,” said Olsen.